Spiny dogfish

Profile

The spiny dogfish is a small shark living off coasts around the world. It inhabits every continent except Antarctica. As you can tell by the range map below, these fish prefer cooler climates and shy away from areas near the equator. The name “dogfish” was given to this animal by fishermen due to the shark’s hunting habits, but we will talk about that more specifically later on. These fish are normally gray but can sometimes have some brown on them. Both of their dorsal fins (the ones on the top of their body) have spines on them that can inject venom. This venom can cause strong pain that lasts for several hours, and in some rare cases it causes death to humans. The spiny dogfish is known by many other names including common spinyfish, piked dogfish, rock salmon, blue dog, grayfish, and white-spotted spurdog.

Size

Female spiny dogfish are larger than the males. The maximum size for males is about 39 inches (1 m), while females can grow up to 49 inches (124 cm). Female’s average weight is around 9.9 pounds (4.5 kg), while the maximum recorded weight is 21.6 pounds (9.8 kg). Occasionally individuals can be as large as five and a half feet (1.6 m), but this is rare.

Diet and hunting

As I said earlier, fishermen gave the dogfish its name because of the way they hunt. Unlike many fish, they hunt in groups, and evidently these groups reminded the fishermen of dogs hunting in packs. Because they are slow swimmers, large numbers really help dogfish capture their prey. Groups can reach into the thousands, and they normally consist of individuals of the same gender and roughly the same size. Main prey includes fish, octopuses, smaller sharks, squid, crabs, and even egg cases of sharks. Fishermen do not like dogfish because of their voracious appetites. These fish have been known to harm local fisheries by consuming many commercially caught fish and scaring many others away. Spiny dogfish have even been observed biting through fishing nets in order to get to the fish. Not only do the fishermen lose the fish that the sharks eat, but the holes in the net allow even more fish to escape.

Habitat

Spiny dogfish Never venture extremely far out to sea. Although they can endure brackish water (a mix of salt and fresh water) these animals prefer complete saltwater and never venture into complete freshwater. This species of shark is commonly found as deep as 2950 feet (900 m) below the surface all the way up to right at the surface. Some studies show, however, that they can dive as deep as 4,080 feet (1244 m)! They are thought to prefer water that is from 44-59 degrees Fahrenheit (7-15 Celsius). During the summer, they live farthest from the equator where the water is more their favorite temperature, but in the winter they migrate closer to the equator were the water is now more suited to their tastes.

Status, threats, and use to humans

The IUCN redlist classifies the spiny dogfish as vulnerable. Although this shark is considered the most abundant living shark, populations in the northern Atlantic Ocean are thought to be at risk due to fishing. In between 1900 and 1910, (the peak of abundance for these animals) it is estimated that 27 million spiny dogfish were caught each year. Some people say that this number is for Massachusetts alone! Currently about 28 million pounds (12.7 million kg) of spiny dogfish are caught each year. Half of this or more is landed in Massachusetts.

There are a lot of ways humans use these sharks. In some places they are caught for their meat and their liver oil. Miscellaneous body parts are also used to make sand paper, vitamins, leather, fertilizer, pet food, and fish meal.

Other predators include larger sharks, seals, killer whales, sea lions, swordfish, and in one case a herring gull.

Mating and young

Mating normally occurs in waters not too far from shore. These sharks have a two year gestation period that is the longest of any vertebrate! After about six months, the membrane that provides nutrients to the unborn sharks breaks down, and the pups get their nutrients from their yolk sac. Usually six or seven young are born, but this number can vary between one and fifteen. When the young are born, a sheath covers each of the spines in order to prevent injury to the mother. At birth, the young are from eight to thirteen inches (20-33 cm) long. One eye-witness of a spiny dogfish birth said that the female was lying upside down and the pups slowly came out head first. Females reach maturity at about twelve years of age when they are about thirty inches (77 cm) long. Males mature at the age of six when they are about 23.5 inches (60 cm) long. There is some controversy about how long these animals live. Some say they can live up to thirty years while others estimate that these sharks can live for up to 100 years!

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